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Re: Vista Support Software
Vladimir Ivanov wrote:
On Sun, 29 Apr 2012, Michael J. Mahon wrote:
Enough time passes during the retries that you can do various "stunts"
with the drive to vary the instantaneous rotational speed--by lifting it
up and rotating it back and forth in a plane parallel to the table--
which is sometimes just what is needed to get a good read! ;-)
I had variable success with slightly opening and closing the door latch,
too. That, of course, when totally in despair.
Interesting! That would slightly change both the clamping force and
the head loading force, which could affect track alignment and speed
respectively.
The really sad thing is that the single byte XOR checksum of the sector
data is simply not enough. When there is a source of noise (including
unreliable data and/or reading process) it is so easy to get "good"
sector which is actually garbage, that it's not even funny.
This is certainly true when the channel is perturbed by random noise.
Fortunately, when a sector is being mis-read due to marginal head
positioning, speed, or very localized media problems, the perturbation
is not random, and the checksum is relatively strong when combined
with field delimiter checks. Put another way, most I/O errors result
in relatively stable (wrong) checksums, not equidistributed checksum
values.
If one were reading a sector which had weak or non-existent recording
over many nibbles, or an equivalent duration of loss of media contact,
then I would expect to see "noise-like" checksums, otherwise, I would
not expect much nondeterminism in the read channel.
Do you have any statistics on mis-reads (of once-valid sectors) that
resulted in good checksums?
It would be interesting to write sectors with a longer (internal)
checksum, then perturb head loading, speed, and alignment and see
what fraction of "good reads" resulted in corrupted data. My purely
anecdotal evidence is that this is quite rare.
Come to think of it, the decades-long experience of Apple II users,
in which the Disk ][ performed as a robust storage medium, confirms
that the one-byte checksum, plus field delimiter checks, delivered
excellent reliability at very low computational cost. If this had
not been the case, the Apple II would have acquired the reputation
of an unreliable computing platform.
-michael
NadaNet 3.1 for Apple II parallel computing!
Home page: http://home.comcast.net/~mjmahon/
"The wastebasket is our most important design
tool--and it's seriously underused."